Apparatus to sputter silicon films

ABSTRACT

A method of physical vapor deposition includes selecting a target material; mixing at least two gases to form a sputtering gas mixture, wherein a first sputtering gas is helium and a second sputtering gas is taken from the gases consisting of neon, argon krypton, xenon and radon; forming a plasma in the sputtering gas mixture atmosphere to sputter atoms from the target material to the substrate thereby forming a layer of target material on the substrate; and annealing the substrate and the deposited layer thereon. An improved physical vapor deposition vacuum chamber includes a target held in a target holder, a substrate held in a substrate holder, a plasma arc generator, and heating rods. A sputtering gas feed system is provided for introducing a mixture of sputtering gases into the chamber; as is a vacuum mechanism comprising at least one turbomolecular pump for evacuating the chamber to a pressure of less than 16 mTorr during deposition. The method and apparatus are particularly suited for forming thin film transistors and liquid crystal displays having thin film transistors therein.

CROSS REFERENCE TO RELATED ADDLICATIONS

This application is a divisional of application Ser. No. 09/576,940,filed May 22, 2000, entitled “Apparatus and Method to Sputter SiliconFilms,” invented by Apostolos Vouteas et al., now U.S. Pat. No.6,429,097.

FIELD OF THE INVENTION

This invention relates to an apparatus and method for the deposition ofthin-film materials used in the fabrication of thin film integratedcircuit devices, and thin film devices made according to the method andwith the apparatus.

BACKGROUND OF THE INVENTION

In the fabrication of thin film (TF) IC devices, such as thin filmtransistors (TFTs), a method and apparatus is needed to form the variouslayers constituting the device. Silicon material, typically amorphoussilicon (a-Si) or polysilicon, are used for the active layers of thedevice and silicon-based insulating layers, typically silicon nitride,SiN_(x), or silicon oxide, SiO_(x), are typically used as insulatorsbetween the active layers. There are several methods to deposit thesefilms. Some methods rely on chemical reactions between one or moresuitable gas-phase species to deposit the silicon-based film on thesubstrate where TF devices are to be fabricated. These reactions requireenergy, which may be supplied in the form of thermal energy, such aschemical vapor deposition (CVD), plasma energy, such as plasma-enhancedCVD (PE-CVD), photon energy, such as photo-enhanced, or laser-pyrolysisCVD, or the presence of a catalyst, as in the case of hot-wire CVD.

Another category of deposition methods are the so-called physical vapordeposition (PVD) methods. In one case, the desired material is depositedby bombarding a suitable “target” of this material with atoms ofsufficient energy, of a typically neutral or inert gas element. Thisprocess is called sputtering and is typically accomplished by farminggas plasma in a gap between a target material and the substrate wherethe material is to be deposited. Argon is the most common gas used inthe sputtering industry. Sputtering is a well-suited method for theformation of the various silicon-based, TF device layers because: (1) itis a safe and environmentally benign technique; (2) it may be used atroom temperature and is therefore compatible with any type of substrate;(3) silicon films with very low H₂ content may be typically depositedand there is no need for dehydrogenation to release excessive hydrogen,or, hydrogen may be incorporated into the film if, and when, necessary;(4) it is a simpler and more easily scaled method than competitivemethods, which rely on chemistry; and (5) it has been successfully usedfor metal depositions in TF devices, such as TFT-LCDs.

There are, however, some problems associated with silicon sputtering.When Ar, at concentrations of>1 at % (atomic percent), is used as thesputtering gas, capture of Ar by the sputtered film causes structuraldefects, which lessens the quality of the deposited film. This phenomenareduces the performance of an amorphous silicon (a-Si) TFT device, andalso causes difficulties in the crystallization of a-Si, a necessarystep in polysilicon TFT technology.

Intrinsic silicon is a resistive material, which, when combined withsputtering in DC mode, leads to the requirement of a significant voltagedrop to maintain a given DC power set point. This problem complicatesthe design of a power supply, increasing the probability of arcing andincreasing the extent of plasma damage on the deposited film due tobombardment by highly energized neutral or charged species.

If He is used as the sputtering gas, to eliminate Ar capture and relatedissues, the deposition rate of the Si or silicon alloy film issignificantly reduced, making it unsuitable for mass production.

Silicon deposition by sputtering has not yet reached manufacturinglevel. As a result, there is no globally accepted solution to theseissues. The main concerns are achievement of a reasonable depositionrate, i.e., >10 Å/s (angstroms/sec), low Ar content and good plasmacharacteristics to reduce plasma damage to the deposited films.

U.S. Pat. No. 5,248,630 to Serikawa et al., granted Sep. 28, 1993, forThin film silicon semiconductor device and process for producingthereof, describes construction of a thin film device includingdeposition of a thin film polycrystalline film.

U.S. Pat. No. 5,665,210 to Yamazaki, granted Sep. 9, 1997, for Method offorming insulating films, capacitances and semiconductor devices,describes deposition of metal oxides and nitride films by RF magnetronsputtering with an atmosphere of less than or equal to 25 at % of inertgas.

U.S. Pat. No. 5,817,550 to Carey et al., granted Oct. 6, 1998, forMethod for formation of Thin Film Transistors on plastic substrates,describes TFT construction on polymer materials.

T. Serikawa, Enhanced step coverage of SiO₂films sputtered inhydrogen-argon mixed gas, compares thin films deposited in a 30% H-70%Ar atmosphere with those in pure Ar. Japanese Journal of AppliedPhysics, Vol. 19, No. 5, May 1980, ppL259-L260.

T. Serikawa et al., Properties of magnetron-sputtered silicon nitridefilms, describes deposition of thin films of 100 nm to 200 nm thicknessat 200° C. from a silicon target in a N—Ar mixture. J. Electrochem.Soc., December 1984, pp2928-2933.

A. Okamoto et al., Magnetron-sputtered silicon films for gate electrodesin MOS devices, describes properties of deposited materials undervarious Ar concentrations. J. Electrochem. Soc., June 1987, pp1479-1484.See FIG. 3.

S. Suyama et al., Electrical characteristics of MOSFETs utilizingoxygen-argon sputter-deposited gate oxide films, describes depositiontechniques at low temperature (200° C.) to form triode MOSFETs. IEEETransactions on Electronic Devices, Vol. ED-34, No. 10, October 1987,pp2124-2128.

T. Serikawa et al., Low-temperature fabrication of high-mobility poly-SiTFTs for large-area LCDs, describes formation of TFTs using laserirradiation. IEEE Transactions on Electronic Devices, Vol. 36, No. 9,September 1989, pp1929-1933.

C. S. McCormick et al., Low temperature fabrication of amorphous siliconthin film transistors by DC reactive magnetron sputtering, J. Vac. SciTechnol. September/October 1997, pp2770-2775.

D. P. Gosain et al., Poly-Si TFT fabrication and hydrogenation using aprocess compatible with plastic substrates, describes TFT formation onpolymer substrates. Electrochem Soc. Proc. vol. 98-22, 1998, pp174-185.

G. K. Giust et al., Low-temperature polysilicon thin-film transistorsfabricated from laser-processed sputtered-silicon films, describes analuminum top-gate coplanar TFT using excimer lasers. IEEE ElectronicDevice Letters, Vol. 19, No. 9, September 1998, pp343-344.

R. T. Fulks et al., Laser crystallized polysilicon TFTs using LPCVD,PECVD and PVD silicon channel materials—a comparative study, comparesthe results of TFT construction by various techniques. Material ResearchSociety, April 1999.

SUMMARY OF THE INVENTION

A method of physical vapor deposition includes selecting a targetmaterial; mixing at least two gases to form a sputtering gas mixture,wherein a first sputtering gas is helium and a second sputtering gas istaken from the gases consisting of neon, argon krypton, xenon and radon;forming a plasma in the sputtering gas mixture atmosphere to sputteratoms from the target material to the substrate thereby forming a layerof target material on the substrate; and annealing the substrate and thedeposited layer thereon.

An improved physical vapor deposition vacuum chamber includes a targetheld in a target holder, a substrate held in a substrate holder, aplasma arc generator, and heating rods. A sputtering gas feed system isprovided for introducing a mixture of sputtering gases into the chamber;as is a vacuum mechanism comprising at least one turbomolecular pump forevacuating the chamber to a pressure of less than 16 mTorr duringdeposition. The method and apparatus are particularly suited for formingthin film transistors and liquid crystal displays having thin filmtransistors therein.

An object of the invention is to provide an apparatus and method offorming thin film devices in an Ar-containing atmosphere whilecontrolling capture of Ar atoms in the thin film.

Another object of the invention is to achieve a reasonable depositionrate, i.e., >10 Å/s (angstroms/sec).

A further object of the invention is to produce a thin film devicehaving a low Ar content.

This summary and objectives of the invention are provided to enablequick comprehension of the nature of the invention. A more thoroughunderstanding of the invention may be obtained by reference to thefollowing detailed description of the preferred embodiment of theinvention in connection with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a PVD chamber constructed according to the invention.

FIG. 2 depicts Si deposition rates as a function of process gas andpressure.

FIG. 3 depicts Ar capture as a function of Ar concentration vs. Arpressure, and compares data from the method of the invention to thatgathered by Okamota et al., supra.

FIG. 4 depicts Si deposition rates as a function of % Ar in the processgas and power level.

FIG. 5 depicts Ar capture and depth profile before and after annealingof the sputtered thin film by excimer laser anneal (ELA).

FIG. 6 depicts Si deposition rate and plasma voltage as a function of(1) %-age of Ar in the sputtering gas and (2) sputtering pressure.

FIG. 7 depicts Si deposition rate as a function of DC power andsputtering gas composition.

FIG. 8 depicts a thin film device constructed according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following describes a method of sputtering, or physical vapordeposition (PVD), for deposition of active layers, such as amorphoussilicon (a-Si) and polysilicon, and deposition of insulating layers,such as SiO_(x) & SiN_(x), and an apparatus for practicing the method ofthe invention. These thin-films are used in the fabrication of thin filmdevices, such as thin film transistors (TFTs), which are most frequentlyused in liquid crystal displays (LCDs), and the invention will bedescribed using TFT construction as an example. One of ordinary skill inthe art will appreciate that the method and apparatus of the inventionmay be used to fabricate other types of TF semiconductor devices.

The problems associated with thin film deposition have been discussedpreviously herein. A solution to those problems involves theintroduction of an appropriate sputtering gas blend to significantlyreduce or eliminate the undesirable effects of Ar, while simultaneouslymaintaining its merits, particularly the deposition rate. As wasmentioned above, use of pure He is not a viable solution to the problemsas the deposition rate of He is approximately 3-4 times slower than thatof Ar, at a given DC power level.

The addition of a small amount of gas to He gas results in a mixyurethat behave more like an Ar gas with respect to deposition rate and morelike He gas with respect to plasma characteristics, such as voltage. Theoptimal window appears to be 3% to 10% Ar in He gas, although mixturesin the range of 1% to 15% Ar in the He/Ar mixture appear to befunctional. Under these conditions, the concentration of Ar in thesilicon film drops below the detection limit of standard Ar measuringmethods, such as secondary ion mass spectrometry (SIMS). Furthermore,the deposition rate of the silicon film remains at 68% to 77% that offilms sputtered by pure Ar. This unexpected behavior is clearly notevident by simple extrapolation of prior art results.

The addition of an optional, low-temperature annealing step, after thefilm deposition, is provided to remove excess He that may have beenincorporated in the film during sputtering. He is much easier to removethan Ar by annealing at temperatures in the range of 100° C. to 300° C.,by furnace annealing or rapid thermal annealing (RTA), compared to theprocess for Ar removal, which requires annealing at temperatures inexcess of 500° C. This optional low temperature anneal may be applied inboth pure silicon and silicon alloy layers, in a-Si and polysilicon TFTapplications.

He may easily be removed during ELA on the sputtered thin film. If theHe-sputtered silicon film is to be laser annealed, the low-temperatureannealing step may not be necessary, as laser annealing is thought to besuitable for the simultaneous removal of He and crystallization of theHe-free, a-Si film to polysilicon. The optional low temperatureannealing step is more appropriate for solid-phase crystallization ofthe sputtered silicon film by, for example, rapid thermal annealing(RTA).

The apparatus of the invention is particularly suited to accomplishingthe thin film deposition according to the method of the invention. Theapparatus includes a vacuum chamber that is pumped by both a cryogenicpump and a turbomolecular pump. FIG. 1 depicts a PVD chamber constructedaccording to the invention generally at 10. Chamber 10 is sealed to theoutside atmosphere and includes a target 12, which is held in place by atarget carrier 13, of material to be deposited on a substrate 14, whichis held in place on a substrate carrier 16. A magnetic field isgenerated by a scanning magnetron 18. Heating rods 20, 22 are providedto adjust the temperature of chamber 10. A sputtering gas feed mechanismprovides the sputtering gases from gas sources 24, 26, and may beintroduced into the chamber independently and mixed in the chamber, ormay be introduced through a sputtering gas mixing manifold which mixesthe sputtering gases prior to their introduction into the chamber.Plasma 28 is generated in the gap between target 12 and substrate 14.Scanning magnetron 18 is used to intensify and confine the plasma.

An important feature of chamber 10 is its vacuum-producing pumpingsystem, shown generally at 30. In the preferred embodiment, two pumpsare attached to chamber 10. In practice, the pumps may be connected toan automated manifold. A cryogenic pump 32 is used to improve the basepressure and reduce the amount of water vapor in the chamber. Aturbomolecular pump 34 is used during processing when He gas is flown,as the cryogenic pump cannot pump He. In an alternate embodiment of theinvention, only a turbomolecular pump is used. The provision of twodifferent types of pumps is based on considerations of improved vacuumquality and faster chamber evacuation in mass production. Because thecryogenic pump is not always in use, its lifetime is extended and itsregeneration frequency reduced. Chamber 10 is capable of producingpressures during deposition or processing of less than 16 mTorr, and aslow as 0.5 mTorr. Chamber 10 may be evacuated to a pressure range of10⁻⁸ Torr. to 10⁻⁹ Torr. when no gases are flown in the chamber.

FIG. 2 depicts of the relationship between silicon film deposition rate,process pressure and process gas. In this particular case, all filmswere deposited at 4 kW and at 400° C. A trace for pure Ar is shown at40, for pure He at 42 and for an Ar/He mixture at 44. As expected, thedeposition rate decreased as the sputtering pressure increased, underconstant power. However, the effect of the process pressure seemed to bemore significant for sputtering in argon than in He. This may beexplained by considering the frequency of gas phase collisions, as afunction of pressure and size of sputtering atoms. At the same pressure,a larger molecule has a higher collision probability.

As the pressure increases, the interaction between smaller mean freepath and higher collision cross-section becomes more important for alarger atom, such as Ar, than a small atom, such as He. Hence, theeffect of pressure is more significant for Ar sputtering. FIG. 2indicates that He has a higher ignition limit in reference to theignition limit of Ar, i.e., He plasma will not ignite at pressures below16 mTorr, 46. The ignition problem with the He gas may be related tosecondary electron ionization phenomena. He is a much smaller atom thanAr; hence, it has a smaller ionization cross-section. In other words,the probability of a collision between an electron and an inert atom, toproduce an ionized atom, is reduced in the case of He. This means that,in order to produce a self-sustained glow discharge, a higher density ofHe atoms is required, which requires a higher operating pressure thanthat required for Ar. It should be further noted that an interestingintermediate case is obtained when Ar and He are mixed together, asillustrated by line 44 in FIG. 2. Not only the deposition rate of themixture is higher than that of “pure” He, but the ignition of themixture tends to follow the characteristics of Ar. The ignition limit ofthe Ar/He mixture is approximately the same as that of pure Ar.

The appropriate pressure range for Si sputtering is determined by theignition characteristics of the sputtering gas and the deposited filmproperties. Data from the apparatus and method of the invention is shownby line 50 in FIG. 3, while that from Okamoto et al., supra, is shown byline 52. FIG. 3 shows that, as the pressure increases, the degree ofincorporation of the sputtering gas, Ar in this example, decreases;i.e., a smaller percentage of the sputtering gas is captured in thesubstrate. This is a desirable property. However, the structuralcharacteristics of the sputtered film also change at higher pressures.The film becomes more porous, and tends to absorb more impurities whenit is exposed to the ambient atmosphere. Thus, the electricalperformance of the film tends to deteriorate at higher pressures. Inaddition, the deposition rate of the film decreases. These are notdesirable properties.

A reasonable compromise between the opposing trends exists in thepressure range of 1-15 mTorr for mixtures of He/Ar. Because of thepresence of Ar in the mixture, plasma may be ignited at low pressures.Even though reduction of both Ar and He content is achieved at thehigh-pressure end, the range of 1.0 mTorr. to 10 mTorr may be effective,and the range of 2.5 mTorr. to 10 mTorr. may be preferable from thepoint of view of film quality and good plasma behavior, as shown in FIG.3 at 54.

FIG. 4 depicts the deposition rate of silicon at various DC powerlevels, as a function of the percentage of Ar in the He/Ar sputteringgas. A number of conclusions may be made from these data. The dependencyof the deposition rate on the percentage of Ar appears to follow twopatterns. In the range of 0% Ar to 25% Ar 56, the deposition rate isstrongly affected by the percentage of Ar in the sputtering gas mixture;in the range of 25%+Ar 58, the deposition rate follows a linearrelationship with respect to the % Ar.

The power level also has a profound effect on this dependency. At lowpower levels, i.e., 1 kW, line 60, the initial range is much smootherand, overall, the deposition rate between pure He and pure Ar sputteringis not very different. Power levels are shown for 2 kW, 62; 4 kW, 64; 8kW, 66, and 10 kW, 68. Above 2 kW however, the transition from pure Heto pure Ar is quite distinct and is associated with a significantincrease in the deposition rate. These data indicate that, as far as DCpower, the regime of interest is above 2 kW for optimum depositionrates.

As far as Ar capture, the significant parameters are: (a) percentage ofAr in the gas feed, and (b) sputter pressure. In addition to the Arcontent, these parameters also affect the sputter rate, the plasmavoltage and the film quality, as pressure affects the film structure.The deposition of silicon with He/Ar mixture was measured at variousoperating pressures. Two pressure regimes appear to exist: 5-6 mTorr and16-17 mTorr. Those of ordinary skill in the art will understand thatwithout any Ar in the mixture, He cannot ignite below 16 mTorr., asshown at 46 in FIG. 2. Hence, the only way to deposit silicon with He ata lower pressure is to add a small amount of Ar gas. A small amount ofAr gas in the film is also thought to provide a more rigid siliconnetwork and improve the mechanical properties of the film. According tothe literature, Ar, even at concentrations as low as 0.2 at % causessignificant retardation to the crystalline growth of silicon. Thenotation “gas % at” indicates percentage of a gas in a film, while thenotation “gas %” indicates the percentage of a particular gas in amixture of gases. It has been speculated that this phenomenon is due tothe formation of Ar bubbles in the film. Such formation will affect thephysical and electrical properties of the film either in the a-Si phaseor in the polycrystalline phase. In the case of alloy deposition, itwill affect the silicon bonding network and, hence, the physical andelectrical properties of insulating films, such as SiO_(x) or SiN_(x).Based on these considerations, an appropriate Ar content range, forcases where Ar content source is to be minimized is: 2·10¹⁸ at/cm³<Ar<2.5·10¹⁹ at/cm³, or, 40 ppm<Ar<0.05%.

There are some cases where the retardation of the nucleation andcrystalline growth in the silicon film are considered positive effects.One example involves the introduction of a catalyst material, such asnickel, at specified locations into the silicon film, to promote nucleiformation and crystalline growth from these preferred locations. This isachieved by subjecting the silicon film to a thermal annealing cycle,thereby enabling the phase transformation from a-Si to polysilicon atlow temperatures via the use of the catalyst. To improve the crystallinequality and its uniformity, it is important to suppress nucleation andcrystalline growth in silicon material void of the catalyst. The way toachieve this is to utilize silicon material that is particularlydifficult to crystallize without an added catalyst. Such material may besputtered silicon with a specified Ar content which is high enough tosuppress partial solid-phase crystallization of silicon material void ofcatalyst. Hence, from this point of view we define the appropriate Arcontent in the range of: 2·10¹⁸ at/cm³<Ar<2·10²⁰ at/cm³.

TABLE 1 Ar concentration in sputtered silicon films Power PressureSputtering Ar content (kW) (mTorr) Gas (at/cm³) 8 14-16 Pure He  <2 ·10¹⁸ He/(3.8%) Ar 2-5 · 10¹⁸ He/(10%) Ar 4-7 · 10¹⁸ Pure Ar  6 · 10¹⁹ 85-6 Pure Ar  2 · 10²⁰

Table 1 shows examples of the Ar content in silicon films sputtered atdifferent pressure and sputtering gas conditions. The Ar content may becontrolled in the ranges identified above, by the deposition pressureand/or the type of sputtering gas. The Ar content in the case of pure Arsputtering is 6·10¹⁹ at/cm³ and is reduced by more than an order ofmagnitude in the case of pure He sputtering, 2·10¹⁸ at/cm³, which is thedetection limit of SIMS analysis. When the He/(3.8%)Ar gas mixture isused, the Ar content in the film is only marginally increased, 2-5·10¹⁸at/cm³. Moreover, this increase may be attributed to noise in themeasurement, as the Ar concentration is very close to the detectionlimit of the measuring technique. Another slight increase in film Arcontent occurs with 10% Ar. Incorporation of Ar, then, is not an issuewhen He/Ar is used as the sputtering gas.

As previously described, excimer laser anneal (ELA) may be used toconvert an a-Si layer to a polysilicon layer. FIG. 5 shows the Arcontent in sputtered silicon films before and after exposure to excimerlaser anneal (ELA) process. Regions of data points are shown for Arcontent with pre-anneal and with ELA, 70, and without ELA, 72; withoutpre-anneal and with ELA, 74, and without ELA, 76. The exposure to ELA,regions 70, 74, results in a reduction in the Ar content in the post-ELAfilms of more than an order of magnitude, with respect to pre-ELA films.In contrast to ELA, subjecting the films to a thermal anneal at 450° C.for three hours in nitrogen ambient produces no effect at all in the Arcontent. Hence, polysilicon films, produced by ELA of PVD amorphoussilicon films, tend to contain Ar at a concentration of about 1-2·10¹⁹at/cm³ or less. Thus, the ELA process is another way to affect theconcentration of Ar in PVD-Si films. This is an important feature of theprocess, because, when the Ar content is required to be initially highto suppress partial crystallization, it may subsequently be reduced byapplying ELA process, so that the quality of the film is improved byeffectively decreasing its Ar content.

FIG. 6 shows the deposition rate, lines 80, 82, and measured plasmavoltage, lines 84, 86, as functions of the % Ar in the mixture and thesputtering pressure. Notice that in a similar fashion to the depositionrate, the plasma voltage is also strongly affected by the consistency ofthe sputtering gas. The addition of a small amount of Ar in the He gasincreases the plasma voltage, but it clearly remains significantly lowerto that corresponding to the case of pure-Ar sputtering. Even with themodest addition of 3.8% Ar in the He gas, the deposition rate is shownto double. Hence, the He/Ar mixture indeed provides an excellentapproach to improve film quality, as determined by a decrease thein-film Ar content and reduced plasma voltage requirement, withoutsacrificing the deposition rate. The 3-10% Ar content regime seems to bethe most promising.

FIG. 7 shows an example of the effect of DC power on the deposition rateof various sputtered silicon films. This comparison illustrates thepower requirements between silicon films sputtered by pure Ar, 90, andthose sputtered by pure He, 92, or by He/Ar mixture, 94, to achieve acertain level of deposition rate, or throughput. The slope of each lineindicates the deposition rate in each sputtering gas in Å/s-kW. Noticethat sputtering by pure Ar results in threefold higher deposition ratethan sputtering by pure-He: 2.18 Å/s-kW versus 0.73 Å/s-kW.

Notice also that when a modest amount of Ar in added in the He gas,i.e., 3.8% Ar in He, the effect on the deposition rate is quitepronounced. For this modest addition of Ar, a more than twofold increasein the rate is obtained: 1.56 Å/s-kW versus 0.73 Å/s-kW. Hence, theHe/(3.8%)Ar gas behaves more like Ar gas when it comes to depositionrate and more like He gas when it comes to plasma voltage, as shown inFIG. 6. Moreover, the deposition rate target of 10 Å/s is easilyattainable with the He/Ar mixture at relatively low DC power levels, 6-7kW, whereas it requires a DC power level of 13-14 kW for pure He. Thisnon-linear behavior of the He/Ar mixtures is an important property,which has not been identified or suggested in previous works.

To deposit a-Si film, hydrogen gas may be also flown, in addition to He& Ar. To sputter SiO_(x), oxygen and/or hydrogen gas may be flown, whilefor SiN_(x), nitrogen and/or hydrogen may be flown, simultaneously withthe He/Ar gas. The reduced plasma voltage, afforded by the He sputteringgas, is expected to be very beneficial for the case of dielectric filmdeposition, such as SiO₂ or SiN_(x). In this case, plasma damage istypically responsible for the introduction of fixed charges in theinsulating films. All of the inert gases, Ne, Ar, Kr, Xe and Rn, may beused in chamber 10, although, as previously mentioned, Ar is by far themost common gas used in PVD. Lower plasma voltage may reduce the plasmadamage and, hence, increase the quality of the dielectric layer.

Referring now to FIG. 8, a liquid crystal display (LCD) apparatus 100includes a lower polarizing plate 102 and an upper polarizing plate 104,which sandwich a liquid crystal (LC) layer 106 therebetween. LC layer106 includes an insulating substrate 108 made of glass or other suitablematerial. Plural gate lines 110 run parallel with each other, and pluralsource lines 112 cross their respective gate lines 110. Lines 110 and112 are formed on insulating substrate 108. Pixel electrodes 114 aredisposed at positions adjacent to respective crossings of gate lines 110and source lines 112, thus forming a matrix on insulating substrate 108.Pixel electrodes 114 are connected to gate lines 110 and source lines112 through TFTs 116 of this example as switching elements.

LCD apparatus 100 further includes an insulating substrate 118 made ofglass or other suitable material, which is disposed so as to opposeinsulating substrate 108. A counter electrode 120 is formed on the innersurface of insulating substrate 118. Insulating substrates 108 and 118are attached together, with liquid crystal contained therebetween, thusforming a liquid crystal layer 106 interposed pixel electrodes 114 andcounter electrode 120. Polarizing plates 102 and 104 adhere to the outersurfaces of insulating substrates 108 and 118. The silicon orpolysilicon layers of LCD 100 and TFTs 116 may be deposited by the PVDmethod of the invention, and will have the characteristics described inconnection therewith.

Thus, a method for deposition of a-Si, polysilicon, SiO_(x) SiN_(x), anddoped silicon alloys by a sputtering process using a gas mixture,including a He/Ar gas mixture, has been disclosed. The invention alsoincludes an apparatus necessary to perform the method of the invention,including a vacuum chamber pumped by combination of turbomolecular pumpand cryogenic pump. Thin film devices manufactured according to theinvention have also been described. It will be appreciated that furthervariations and modifications thereof may be made within the scope of theinvention as defined in the appended claims.

We claim:
 1. An improved physical vapor deposition vacuum chamber, atarget held in a target holder, a substrate held in a substrate holder,a plasma arc generator, and heating rods, the improvement comprising: asputtering gas feed system for introducing a mixture of sputtering gasesinto the chamber, the sputtering gases including helium; a vacuummechanism including a cryogenic pump for evacuating the chamber to abase pressure before sputtering gases are introduced into the chamber;and the vacuum mechanism further including at least one turbomolecularpump for evacuating the chamber to a pressure of less than 16 mTorrduring deposition.
 2. The improved physical vapor deposition vacuumchamber of claim 1 wherein the base pressure said vacuum mechanismachieves by means of said cryogenic pump before sputtering gases areintroduced into the chamber is in the range of 10⁻⁸ Tort to 10⁻⁹ Torr.3. The improved physical vapor deposition vacuum chamber of claim 1wherein said feed mechanism includes separate feed lines for eachsputtering gas.
 4. The improved physical vapor deposition vacuum chamberof claim 1 wherein said feed mechanism includes a sputtering gasmanifold for mixing said sputtering gases prior to introduction of thesputtering gas mixture into the chamber.